In the fabrication of vertical, high performance bipolar transistors, it is generally desirable to provide very shallow intrinsic base regions. Such shallow intrinsic base regions, preferably in the sub-micron dimension range, permit the fabrication of extremely high-frequency transistors: i.e., having speeds in the gigahertz switching range.
The formation of the intrinsic base region in the transistors described above is typically accomplished by diffusion from a gaseous or solid diffusion source, or by ion implantation (I/I). Diffusion tends to be difficult to control, and may result in undesirably thick base regions. Ion implantation, while more controllable than diffusion, also has inherent limitations. Particularly with respect to ion implantation of boron (B) atoms, the thickness of the implanted region is limited by secondary channeling effects. Further, both diffusion and ion implantation are particularly difficult to utilize with transistors having shallow, heterojunction base regions: e.g. silicon-germanium (SiGe) base regions.
A further challenge faced in manufacturing transistors with narrow intrinsic base regions is that of providing a reliable electrical connection to the intrinsic base region. Such connections are typically accomplished through the use of an extrinsic base region--a thick, highly doped region disposed in contact with the edge of the intrinsic (or thin, active) base region. As the intrinsic base region decreases in thickness, the linkup between extrinsic and intrinsic base regions becomes more difficult to reliably establish. One typical fault encountered in making such a linkup is that of the extrinsic base and emitter regions butting so as to cause an unacceptably low base-emitter breakdown voltage.
With respect to the formation of thin, intrinsic base regions, a low temperature, ultra-high vacuum, chemical vapor deposition (UHV/CVD) process is known in the art for forming thin, discretely doped layers of epitaxial silicon. See Meyerson, B., "Low Temperature Silicon Epitaxy by Ultrahigh Vacuum/Chemical Vapor Deposition," Appl. Phys. Lett. 48(12), 24 Mar. 1986, pgs. 797-799. This process has also been used to form SiGe layers, as well as various device regions, including, in certain limited configurations, the base regions of transistors. The process is advantageous for these purposes in that it provides relatively defect-free, thin layers. Intrinsic base regions formed using this process, however, may be particularly difficult to connect with the extrinsic base.
Further known in the art is the use of a linker region for electrically linking an intrinsic base region to an extrinsic base region. A linker region, typically a region more lightly doped than the extrinsic base region, is formed proximate the junction between the extrinsic and intrinsic base regions. The purpose of the linker region is to insure a good electrical connection between the two regions. Linker regions are particularly useful where the spacing between the intrinsic and extrinsic base regions may vary, as, for example, where a sidewall spacer is used to set such spacing.
With respect to linker regions, Sugiyama, M., et al , "A 40GHz f.sub.T Bipolar Transistor LSI Technology," IEDM 89 221-224, shows a linker region out-diffused from a borosilicate glass (BSG) sidewall spacer, the spacer subsequently functioning as an insulator. The use of BSG as a diffusion source is particularly difficult to control, and thus this process is unlikely to yield high performance transistors.
Japanese Kokai No. 62-293,674 (1987) by Matsushita Electric Industrial Co., Ltd., while not showing linker regions, shows a vertical bipolar transistor wherein an intrinsic base region is formed by implanting ions into a recessed surface intermediate bounding, highly doped, extrinsic base regions. The recessed surface, formed by etching after the extrinsic base regions have been formed, results in the removal of inner portions of the extrinsic base regions. The intrinsic base region is subsequently formed by ion implantation. As discussed above, the use of ion implantation results in an intrinsic base region having a minimum thickness inherent in the process limitations. In the particular structure shown, the extrinsic base region is in direct contact with the emitter region, which will likely result in substantially degraded performance. Further, because the etching of silicon is difficult to control, the basewidth and corner doping profile of the transistor will be difficult to control.
In summary, while high performance, vertical bipolar transistors are known which provide very fast switching speeds, several technical problems need to be addressed in order to continue to advance the art. In particular, the present inventors have recognized the need to provide improved methods of forming ultra-thin intrinsic base regions, and the need to provide reliable electrical connections to such thin intrinsic base regions.